Air data sensing probes are utilized in aircraft to sense various air properties such as total pressure, static pressure, total temperature, etc. A major problem with conventional air data sensing probes is that ice builds up on the probe, which in turn, may lead to false reading or may temporarily cause the probe to stop functioning. As the pilot of the aircraft must rely upon the continuous and proper functioning of the instrumentation of the aircraft, poorly functioning or malfunctioning air data sensing probes are unacceptable.
In order to de-ice air data sensing probes, a number of approaches have been taken. For instance, conventional resistive heaters have been utilized. Heaters have traditionally used one heating element with or without temperature control.
One type of heater utilized in conjunction with air data sensing probes is a resistive type heater such as a metal core heater. However, there are a number of disadvantages inherently associated with utilizing metal core heaters. For instance, a metal core heater utilized without temperature control operates at high temperature, and is therefore relatively unreliable because they frequently burn out. A metal core heater used with linear temperature control usually has an associated electronic module. However, electronic modules have poor efficiency, add cost and reduce reliability of the overall system. Alternatively, a metal core heater may be used with an “on/off” temperature control. However, these too contain electronic modules that generate high current noise on power lines, add cost and reduce reliability of the overall system. Another problem related to resistive type heater is that the associated control element requires additional space, which is severely limited on an aircraft. Therefore, heating systems that require additional electronic controls for proper operation are undesirable.
Another type of heater that may be used in conjunction with air data sensing probes is a solid-state heater. Solid-state heaters have the advantage of inherent temperature control. In addition, solid-state heaters are relatively compact and small in size. However, a major disadvantage to use of a single solid-state heater element is that solid-state heaters are notoriously unreliable, having a relatively short lifespan prior to failing or burning out. Therefore, traditionally solid-state heater elements have not been utilized.
A number of patent have sought to provide heating system for air data sensing probes, however none of these have provided an effective and highly reliable de-icing system.
One such system is disclosed in U.S. Pat. No. 5,464,965 to McGregor (“the '965 patent”). The '965 patent discloses the use of a controller for regulating the temperature of air data probes with a resistive element such that the resistance of the element varies with temperature. Although the space required for the associated electronics is reduced, associated circuitry is still required, which in turn requires additional space. The additional circuitry performs analog calculation base on the instantaneous applied voltage and current returned through the resistive element. Another disadvantage of the system disclosed in the '965 patent is that it teaches the use of resistive type heaters which suffer from the inherent problems discussed above.
Another system is disclosed in U.S. Pat. No. 4,458,137 to Kirkpatrick (“the '137 patent”). The '137 patent discloses use of an air data sensor device having a first resistive heater connected in series with a positive temperature coefficient (“PTC”) resistive heater and a second resistive heater connected in parallel with the first resistive heater so as to increase the system heating capacity. Although the control circuitry for the '137 patent will be reduced by utilization of a PTC heater, addition control circuitry is necessary for control of the resistive type heater which will require additional space. Although the '137 patent discloses the connection of the resistive type heaters in parallel so as to generate more heat, this will not increase the lifespan or reliability of any of these resistive heaters. Rather, the '137 patent is directed toward solving the problem of avoiding excessive heating of the probe itself thereby causing damage to the sensor rather than the heating elements. The '137 patent accomplishes this by regulating the resistive heaters based upon the air temperature so as not to overheat the probe, however nothing is disclosed with regard to extending the life of, increasing the reliability of, or reducing the cost of maintaining the heater elements themselves. A further limitation of the '137 patent is use of a PTC heater. PTC heaters are self-regulating with temperature, however they are characterized with having a linear response to temperature change and are limited to approximately a 1 to 4 heating ratio. For instance, if a 40 watt heater is utilized, the PTC heater will range between 10-40 watts based upon the temperature. The heating capacity of PTC heaters also limited, such that standard resistive type heaters are utilized in connection with them to increase heating capacity as disclosed, for instance, in the '137 patent.
Still another system is disclosed in U.S. Pat. No. 4,121,088 to Doremus et al. (“the '088 patent”). The '088 patent discloses a PTC resistive heater made up of a plurality of individual PTC resistors connected electrically or mechanically in parallel by flexible electrically conductive perforated strips. Individual heater resistors are encapsulated and connected in parallel to accommodate different coefficients of expansion between the heater material and the sensor material so as to avoid overheating of the probe casing so as not to crack or damage it. The use of a PTC restive heater is disclosed where the resistance of the heating elements varies with temperature. However the use of parallel connected resistive type heater, as disclosed in the '088 patent is designed to minimize damage caused to the probe itself due to overheating of the heating elements. The system disclosed in the '088 patent will not extend the life of the heating elements themselves to reduce maintenance costs and increase reliability.
A further system is disclosed in U.S. Pat. No. 4,000,647 to Tauchmann (“the '647 patent”). The '647 patent discloses a thermal controlled resistor means that exhibits a low increase of resistance within a specific temperature range and a high increase in resistance within an adjacent range. However the use of a thermally controlled resistive type heater, as disclosed in the '647 patent, requires the use of additional control circuitry which introduces the inherent problems discussed above and requires additional space to house the control circuitry.
Therefore, what is desired is a highly reliable heating system for de-icing an air data sensing probe.
It is also desired to provide a de-icing system for air data sensing probes that avoids the inherent problems associated with traditional resistive type heaters.
It is further desired to provide a de-icing system for air data sensing probes that will function for an extended period of time without need of replacement.
It is yet further desired to provide a de-icing system for air data sensing probes that requires a minimal amount of space.
It is also desired to provide a de-icing system for air data sensing probes that minimizes the cost associated with maintaining the de-icing system in operable condition.